Share a small trick of Firefox. Sometimes on Firefox, we could not visit a page, it apparently will show an error page. It is normal to us.

But some of you might prefer an alert pop up than an error page. So, just in your firefox config (type about:config on the address bar and enter). Search for browser.xul.error_pages.enabled set it False for a pop up.

The reason of popup is because it is much faster and lesser memory consumption for a page rendering. However, it will become annoying if there are plenty alert boxes pop up at the same time.